1. Field of the Invention
The present invention relates to a controlling system and method for controlling an electrochromic element, and more particularly in to a controlling system and method using a fixed frequency for driving a reflectance of an electrochromic element.
2. Description of the Prior Art
As known, various electrochromic elements, i.e. electrochromic mirror and window systems applied in automotive applications, are being widely used. Relatively, several apparatus for controlling the reflectance of said electrochromic elements are progressively developed. For example, U.S. Pat. No. 6,084,700 as illustrated in FIGS. 1 and 2, discloses that an electrical circuit utilizes active loading or voltage feedback of electrochromic element to control a reflectance of an electrochromic element. The electrical circuit provided a programmable microcontroller is capable of generating a variable duty cycle driver to adjust the current voltage of electrochromic elements. In illustration of FIG. 1, the microcontroller gets an element voltage with a PWM (pulse width modulated signal) input voltage during measurement and therefore looks up a corresponding output voltage target pre-stored in a lookup table of the microcontroller. Furthermore, the microcontroller compares the output voltage target with said element voltage to increase/decrease the PWM input duty cycle as forming the variable output duty cycle. The flowchart for software PWM installed in the microcontroller is illustrated in FIG. 2. However, there are several problems in use of the variable output duty cycle for electrochromic element. It is over complexity in design and therefore possibly makes unreliable.